The present invention relates to a method of feedback-controlling the air-fuel ratio of an internal combustion engine, and more particularly, it relates to a method of this kind wherein the air-fuel mixture supplied to the engine is feedback-controlled to a desired air-fuel ratio in response to the output of an exhaust gas ingredient concentration sensor having output characteristics in approximate proportion to the exhaust gas ingredient concentration, and the fuel supply to the engine is cut off when the engine is in a predetermined deceleration region.
In methods for feedback-controlling the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine (referred to hereinafter as "supply air-fuel ratio") to a desired air-fuel ratio in response to the output of an exhaust gas ingredient concentration sensor having output characteristics proportional to the exhaust gas ingredient concentration, the desired air-fuel ratio is set according to operating conditions of the engine.
However, when the fuel supply is cut off (referred to hereinafter as "fuel cut") when the engine is in a predetermined deceleration region and the fuel supply is re-started upon termination of fuel cut, there is a problem that if the desired air-fuel ratio is immediately set to a value depending on an operating condition in which the engine is operating, upon re-starting of the fuel supply, feedback control cannot be properly performed based on the difference between the desired air-fuel ratio and the air-fuel ratio detected by the sensor, due to a response delay in the control system and other factors.
In order to resolve this problem, a method has been disclosed (Japanese Provisional Patent Publication No. 2-11842 (Kokai)) wherein the desired air-fuel ratio is immediately set upon termination of fuel cut to a leaner value than the proper value which should be set on the basis of the engine operating condition, and is then progressively increased to a richer value.
If the fuel cut period is relatively short, however, there is effectively no change in the engine operating condition. If the fuel cut period is relatively long, on the other hand, the engine operating condition does change such that in some cases it is better to adjust the air-fuel ratio to a richer value immediately when the fuel supply is restarted. According to the aforesaid method wherein the desired air-fuel ratio is progressively increased from a lean value to a rich value irrespective of the engine operating condition during the fuel cut period, it was sometimes impossible to adjust the system rapidly enough to obtain the desired air-fuel ratio.